The present invention relates generally to superconducting generators and more particularly to means for and a method of protecting certain components of the generator during particular abnormal conditions.
A typical superconducting generator of the general type to which the present invention is directed includes a number of conventional components. One such component is a supercooled rotor which includes superconducting field windings and a structure for supporting these windings. The rotor is supercooled to a cryogenic temperature by a fluid refrigerant, specifically liquid helium, which is contained within the rotor. Other conventional components of this generator include a pair of torque tubes respectively connected at their ends to opposite ends of the rotor support structure, a torque tube heat exchanger associated with each torque tube and a thermal radiation shield, specifically a cylindrical shield which extends coaxially around the rotor.
During normal operation of the generator, the liquid helium within the rotor is reduced to a gas through boil-off at a relatively low but constant rate. This helium gas is directed to and along the length of each torque tube by its associated heat exchanger. These torque tubes function to transmit torque from the generator driver which is at room temperature to the winding support structure of the rotor which is at liquid helium temperature (about 3.5.degree.-4.2.degree. Kelvin (K). At the same time, they limit thermal stress and cold end heat leakage due to temperature gradients.
The function of the radiation shield is to intercept heat radiation from its ambient surroundings which is typically at room temperature so as to prevent the radiated heat from warming the cryogenic cold zone of the generator, specifically the area surrounding the inner rotor. Thermodynamic considerations have heretofore indicated that a single radiation shield should operate at about 100.degree. K. in order to minimize liquid helium refrigeration costs. However, when one realizes that 100.degree. K. is -280.degree. F., it should be quite apparent that the shield itself must be supercooled. In copending U.S. application patent Ser. No. 905,042, which was filed on May 11, 1978, and assigned to the present Assignee and which is entitled RADIATION SHIELD FOR USE IN A SUPERCONDUCTING GENERATOR OR THE LIKE AND METHOD, a supercooled radiation shield is disclosed. As described in this application, a plurality of passageways are provided internally through the shield and continuous streams of helium gas are applied through these passageways during normal operation of the generator for supercooling the shield internally.
The superconducting generator just recited has been described during normal operation. However, where this type of generator is intended for use in power plant applications, it must be designed to survive the most severe operating condition in such a system, specifically the three phase high voltage transmission line fault. During such a fault, electromagnet losses occur in the field windings comprising part of the supercooled rotor and also in the radiation shield and rotor support structure. This, in turn, causes the liquid helium within in the rotor to boil off at a substantially higher rate which, in turn, causes the flow rate of the helium gas through the torque tube heat exchangers to increase substantially, thereby causing the temperatures of the torque tubes to drop significantly. In fact, it has been predicted that in typical superconducting generators of the general type described above, a helium flow rate would be sufficiently high and the drop in temperatures of the torque tubes would be sufficiently drastic to cause the torque tubes to fracture under these abnormal operating conditions.
One suggested way to prevent fracturing the torque tubes during a fault causing abnormal operation of the type described, is to utilize a flow dividing mechanism for passing the helium stream out of the rotor and directly to an external heat exchanger at the onset of the fault, bypassing the torque tube heat exchangers altogether. While this approach protects the torque tubes during a three phase high voltage transmission line fault, it requires costly and sometimes unreliable flow dividing valves and external heat exchangers. On the other hand, as will be seen hereinafter, the present invention provides a way of protecting the torque tubes utilizing a thermal radiation shield of the type described in the previously recited copending application. As will also been seen, the approach disclosed herein is one which is uncomplicated in design, reliable in use and economical to provide.